Xenopus laevis is one of the best ectothermic vertebrate models for studying the phylogeny and ontogeny of the immune system. The evolutionary distance of X. laevis from mammals permits distinguishing species-specific adaptations from more conserved features of the immune system. X. laevis, provides a unique, versatile, non-mammalian model with which to study humoral and cell-mediated immunity in the context of MHC restricted and unrestricted recognition, ontogeny, and phylogeny, and against tumors, viruses, and bacteria. In particular, the developmentally regulated acquisition of MHC class I molecules during metamorphosis and the ease with which one can experimentally manipulate embryos and larvae prior to expression of class I and other adult-specific antigens allows one to address questions about MHC restriction, autoimmunity, and the development of self-tolerance that can not be easily studied in other animal models. Studies with X. laevis over several decades have resulted in the generation of many invaluable research tools including MHC-defined isogenetic clones and inbred strains of animals, transgenic lines, cell lines, monoclonal antibodies, and cDNA probes that need to be preserved, enriched and made available to the scientific community. The broad objective of this renewal proposal, therefore, is to safeguard, promote and further develop X. laevis as an important model for biomedical research in general, and immunology, in particular. As in the original proposal, two major aims are proposed: (1) Maintenance, improvement and advertisement of our X. laevis facility by continuing to maintain and improve the performance and quality of our resource;by providing animals, not commercially available, and reagents upon request;by assisting, training, and informing scientists and students about X. laevis;and by disseminating information, advertising and interacting with the scientific community through a web site. (2) Development of new experimental animals, methodologies, and reagents by producing a new collection of isogenetic clones MHC and minor-H-antigen defined;by adapting transgenesis techniques and generating transgenic, which are isogenetic clones, expressing fluorescent reporter genes in lymphoid tissues (e.g., thymus, spleen) or leukocytes;by developing in vivo knockdown by RNA interference using transgenesis to reveal the function of immunologically-relevant genes;and by generating new X. laevis-specific antibodies (Abs) recognizing immunologically-relevant molecules. RELEVANCE: The overall objective of this renewal application is to safeguard and promote the frog Xenopus laevis, as an important non-mammalian comparative model for biomedical research in general, and immunology, in particular. We are proposing to continue the maintenance and the development of this unique non-mammalian resource facility for biomedical research for the benefit of the whole scientific community.